Polyester Conjugated Yarn Having High Self-Crimping Properties and Method of Manufacturing the Same

ABSTRACT

Disclosed is a polyester conjugated yarn, including two types of polymers having a large intrinsic viscosity difference subjected to bi-component spinning so as to be longitudinally arranged in a side-by-side sectional structure, in which the twp types of polymers are polyethyleneterephthalate having intrinsic viscosity of 0.45-0.65 as a first polymer and polytrimethyleneterephthalate having intrinsic viscosity of 0.90-1.10 as a second polymer. The polyester conjugated yarn is advantageous in terms of superior spinnability and high uniformity index. In addition, the polyester conjugated yarn can exhibit high self-crimping properties through relaxation-heat treatment of the dyeing and finishing process, and as well, can be applied to manufacture woven/knit fabrics having softness to the touch, beautiful colors, and high drapery and bulk properties, due to inherent characteristics of polytrimentyleneterephthalate.

TECHNICAL FIELD

The present invention relates to a polyester conjugated yarn, including two different types of polymers subjected to bi-component spinning so as to be longitudinally arranged in a side-by-side sectional structure, and a method of manufacturing the same.

BACKGROUND ART

Generally, it is known that a conventional side-by-side type conjugated yarn composed of two types of polymers has self-crimping properties, and thus, exhibits uniform spiral crimp tendency by a relaxation-heat treatment during a relaxing process in dyeing and finishing process, as in a bimetal having two metals with different thermal expansion coefficient.

Until now, to manufacture a conjugated yarn having self-crimping properties, there have been used methods of single spinning with one type of polymer by means of varying the quench air velocity, and methods of bi-component spinning with two different types of polymers. In such cases, the crimp properties of the single spinning method using the quenching is inferior to those of the bi-component spinning method. Also, the single spinning has the difficulty of mounting the manufacturing equipment in a large scale. Hence, the bi-component spinning method using the two types of polymers is presently commercially employed.

In this regard, examples of selecting polymer pairs include the same polymers being different in only intrinsic viscosity, for instance, I.V. 0.65 regular polyester as a first component and I.V. 0.46 regular polyester as a second component, and the same kind of polymers having different shrinkage properties, for instance, general polyester as a first component and copolymerized polyester having high shrinkage as a second component. However, use of the copolymer, which is generally low in physical properties, results in low spinnability and reduced crimping properties of the conjugated yarn. Further, in the cases of using the difference of intrinsic viscosity of the same polymers, there is required a specific spinnerette, and it is difficult to mass-produce a conjugated yarn.

On the other hand, yarn prepared using two kinds of polymers having different shrinkage properties so as to exhibit high crimping properties and high elasticity, that is, yarn resulting from conventional methods disclosed in many patents, is disadvantageous in that the polymers should be subjected to two stages, for example, spinning at a slow rate (1000-1500 m/min) or a fast rate (2500 m/min or more), and drawing by use of a drawing machine having a first godet roller of 80-120° C. and a thermosetting part of 180-250° C., whereby heat shrinkage difference between the polymers may be caused by relaxation-heat treatment using dry heat or moist heat in the dyeing and finishing process, thus crimps are formed.

DISCLOSURE OF THE INVENTION

Leading to the present invention, the intensive and thorough researches on side-by-side conjugated yarns, carried out by the present inventors aiming to avoid the problems encountered in the related art and to solve drawbacks, such as bending or kneeling on the nozzle surface, caused by the difference of intrinsic viscosity between the polymers, lead to development of a specific spinnerette being capable of performing a bi-component spinning process of polymer pairs having different intrinsic viscosities. Further, to maximize the difference of intrinsic viscosity between the polymers, polytrimethyleneterephthalate, which has superior spinnability and elasticity and exhibits good dyeability at low temperatures and softness to the touch, with higher intrinsic viscosity, compared to polyethyleneterephthalate, is used. Thereby, polyethyleneterephthalate and polytrimethyleneterephthalate are subjected to side-by-side bi-component spinning, thus obtaining a desirable polyester conjugated yarn having high self-crimping properties with a circular section. As such, even though the conjugated yarn is manufactured not by conventional two-staged process composed of spinning and drawing but by a one-staged spindraw process of the present invention, it can exhibit high self-crimping properties upon relaxation-heat treatment of the dyeing and finishing process. As well, inherent characteristics of polytrimethyleneterephthalate can result in high softness to the touch, beautiful colors, and superior drapery and bulk properties of woven/knitted products. Also, the polyester conjugated yarn can have high spinnability and uniformity index.

Accordingly, an object of the present invention is to provide a side-by-side type polyester conjugated yarn.

Another object of the present invention is to provide a method of manufacturing such a polyester conjugated yarn.

To achieve the above objects, the present invention provides a method of manufacturing a polyester conjugated yarn, including: subjecting two types of polymers having a large intrinsic viscosity difference to bi-component spinning in a spindraw manner by the use of an inclined circular spinnerette so as to cause the polymers to have a side-by-side sectional structure, the two types of polymers being polyethyleneterephthalate with intrinsic viscosity of 0.45-0.65 as a first polymer and polytrimethyleneterephthalate with intrinsic viscosity of 0.90-1.10 as a second polymer,

wherein the polyester conjugated yarn has a cross section satisfying Equations 1 and 2, below: 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2

in which,

line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer;

line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2

line EF: a maximum length of a long axis of a cross section of a yarn;

line GH: a maximum length of a short axis of a cross section of a yarn.

Additionally, the present invention provides a polyester conjugated yarn, including two types of polymers having a large intrinsic viscosity difference subjected to bi-component spinning so as to cause the polymers to have a side-by-side sectional structure, the two types of polymers being polyethyleneterephthalate with intrinsic viscosity of 0.45-0.65 as a first polymer and polytrimethyleneterephthalate with intrinsic viscosity of 0.90-1.10 as a second polymer,

wherein the polyester conjugated yarn has a crimp ratio not less than 20% and a circular cross section satisfying Equations 1 and 2, below: 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2

in which,

line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer,

line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2

line EF: a maximum length of a long axis of a cross section of a yarn;

line GH: a maximum length of a short axis of a cross section of a yarn.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of a spinning machine used for a manufacturing method of the present invention;

FIG. 2 is a schematic view of a bi-component spinning pack used for the present invention;

FIG. 3 is a schematic view of a conventional straight spinnerette;

FIG. 4 is a schematic view of a cross section of a polyester conjugated yarn manufactured by the present invention; and

FIG. 5 is a schematic view of a modification of a cross section of a polyester conjugated yarn, according to manufacturing conditions of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Useful for a side-by-side type bi-component spinning process of the present invention, polyethyleneterephthalate (hereinafter, abbreviated to ‘PET’) as a first polymer has intrinsic viscosity of 0.45-0.65, while polytrimethyleneterephthalate (hereinafter, abbreviated to ‘PTT’) as a second polymer has intrinsic viscosity of 0.90-1.10. As such, it is preferred that the above two types of polymers are largely different with regard to intrinsic viscosities. On the other hand, when a general circular straight spinnerette (FIG. 3) is applied for a spinning pack, melt viscosity becomes different due to the difference of intrinsic viscosity directly under the nozzle upon a spinning process. Thus, yarn is bent toward the polymer having high melt viscosity, thus causing a bending or kneeling phenomenon, which negatively affects spinnability and mass production. Therefore, developed by the present inventors, an inclined circular spinnerette (FIG. 2) is applied for a conjugate spinning pack 3 of FIG. 1 so as to offset the difference of melt viscosity and to ensure stable spinnability without the bending of the yarn.

According to the present invention, the first polymer having low intrinsic viscosity and a high flow rate is spun through a surface A of the inclined circular spinnerette, and the second polymer having high intrinsic viscosity and a low flow rate is spun through a surface B of the inclined circular spinnerette. Thus, the two types of polymers having larger difference of intrinsic viscosity can be stably spun without the bending of the polymers to be discharged, by use of the above inclined circular spinnerette. In such cases, it is important that the two types of polymers have the melt viscosity difference of 1500 poise or more. The larger the viscosity difference, the more favorable the spinning process. However, if the viscosity difference exceeds 2500 poise, the yarn is severely bent toward the high viscosity polymer, and thus, it is impossible to perform the spinning process. Meanwhile, the melt viscosity difference less than 1500 poise results in drastically bent polymers toward the surface A having a high flow rate, thus reducing spinnability.

The PET as the first polymer is composed of terephthalic acid and ethyleneglycol as first constitutive monomers, while the PTT as the second polymer includes terephthalic acid and propanediol as second constitutive monomers, in which the resulting polyester does not contain a third functional component copolymerized, The melt viscosity difference between the two polymers is controlled by varying thermal hysteresis of each molten polymer according to different temperature conditions of extruders 1 and 1-1 shown in FIG. 1 of each polymer component, or by adjusting spinning temperatures of a high viscosity polymer or a low viscosity polymer. With the aim of the melt viscosity difference of the two polymers being properly maintained in the level of 1500-2500 poise upon spinning, the spinning temperatures are preferably adjusted in the range of 265-290° C. Therefore, it is preferred that the extruder 1 of PET is in the temperature range of 275-295° C., and the extruder 1-1 of PTT is in the temperature range of 250-270° C. The molten polymer passed through each extruder are fed into a spinning pack 3 through gear pumps 2 and 2-2.

The yarn spun by the polymers having the melt viscosity difference has a circular cross section as shown in FIGS. 4 and 5, in which an interface between the high viscosity polymer and the low viscosity polymer is formed to be round due to the melt viscosity difference therebetween. As such, the high viscosity polymer has a convex interface in section, while the low viscosity polymer has a concave interface in section. The shape of the interface is changed in the range satisfying the following Equation 1 by the melt viscosity difference: 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1

wherein,

line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer;

line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2

In addition, the PET constitutes 30-70 wt % and the PTT constitutes 70-30 wt %, based on total weights of the polyester conjugated yarn. Further, the shape of the cross section of the yarn is changed in the range satisfying the following Equation 2 by the different throughput ratios: 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2

wherein,

line EF: a maximum length of a long axis of a cross section of a yarn;

line GH: a maximum length of a short axis of a cross section of a yarn

In the present invention, a winding speed 6 amounts to 3000-5500 m/min, which is not particularly limited. Upon spinning of a yarn by means of a one-staged spindraw process, a speed of a first godet roller 4 is set to 2000 m/min or more, and a speed of a second godet roller 5 is set to 4000 m/min or more, thereby increasing spinnability and crimp properties. When a drawing temperature of the first godet roller is too low, uneveness is caused upon a dyeing process. On the other hand, if the above drawing temperature is too high, a yarn pass of godet roller is unstable, thus process performance is poor. Hence, the first godet roller is preferably in the temperature range of 70-100° C. In addition, if the second godet roller has too low a temperature, settability of yarns becomes inferior, and thus, the resultant yarn has many defects when being formed into woven/knit fabrics through the dyeing and finishing process. Meanwhile, the extremely high temperatures of the second godet roller result in an unstable yarn pass on godet roller, which negatively affects a spinning process. Accordingly, the second godet roller is preferably set to be in the range of 100 to 140° C.

The conjugated yarn of the present invention has a strength of 2.0-3.3 g/denier, and an elongation of 20-40%. When the strength of the yarn is less than 2.0 g/denier, yarn breakage phenomena increase during the spinning process, and the process performance decreases upon the production of woven/knit fabrics. Eventually, the woven/knit fabrics have low tear strength. Whereas, when the strength exceeds 3.3 g/denier, the manufactured woven products are poor in the sense of touch. In addition, the elongation less than 20% results in easy generation of fluff upon yarn package, while the elongation exceeding 40% leads to a low uniformity index (U %) due to the unstable yarn pass in the spinning process.

In the present invention, the polyester conjugated yarn having high self-crimping properties is composed of polyethyleneterephthalate as the first polymer and polytrimethyleneterephthalate as the second polymer, and has a crimp ratio of 20% or more. Also, the above conjugated yarn has a circular cross section, which satisfies the following equations (FIGS. 4 and 5): 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2

wherein,

line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer;

line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2

line EF: a maximum length of a long axis of a cross section of a yarn;

line GH: a maximum length of a short axis of a cross section of a yarn.

Moreover, as for the conjugated yarn of the present invention, a crimp ratio, measured by a procedure as stated later, is preferably not less than 20%. That is, when a side-by-side type conjugated yarn of PET and PTT has a crimp ratio of 20% or more, it can be applied to the production of the woven/knit fabrics having desired elasticity. If the crimp ratio is less than 20%, elasticity of the woven/knit fabrics decreases. Also, the conjugated yarn having self-crimping properties, which has the circular cross section satisfying the above equations, has a larger interface at which the two types of polymers are in contact with each other, compared to other conjugated yarns with non-circular cross sections. Hence, the circular conjugated yarn has great numbers of crimps per unit length through relaxation-heat treatment of the dyeing and finishing process, which cause the increase of elasticity of the woven/knit fabrics. As well, when such a yarn having circular cross section is fabricated into the woven/knit fabrics, the circular cross section thereof causes soft and comfort wearing.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

EXAMPLES 1-6

An inclined circular spinnerette was used for a side-by-side type bi-component spinning process of PET and PTT with a maximized difference of intrinsic viscosity. As a polymer A, PET having intrinsic viscosity of 0.460, 0.550 and 0.635 was used, and PTT having intrinsic viscosity of 1.00 was used as a polymer B, as shown in Table 1, below. Further, under spinning temperature conditions at 270-290° C. and different throughput ratios as in Table 1, below, a conjugated yarn was manufactured through a one-staged process by use of the inclined circular spinnerette. A cooling air of 23° C. at 5-120 cm directly under the nozzle was fed at 0.35 m/sec, and 0.5-1.1 wt % of a spinning oil was used. The resultant yarn was woven in warp and well directions to manufacture a woven fabric of 100 g/m², which was then dyed at 120° C. TABLE 1 Spinning Visco. Ex. Polymer Polymer Temp. Differ. throughput Interface Shape No. A B (° C.) (poise) Ratio ratio ratio Process 1 PET 0.635 PTT 1.00 290 1700 5:5 0.20 1.03 One-staged 2 PET 0.550 PTT 1.00 278 2120 5:5 0.24 1.04 One-staged 3 PET 0.550 PTT 1.00 285 2000 6:4 0.25 1.06 One-staged 4 PET 0.550 PTT 1.00 288 1960 4:6 0.52 1.08 One-staged 5 PET 0.460 PTT 1.00 275 2800 5:5 0.35 1.07 One-staged 6 PET 0.460 PTT 1.00 290 2100 5:5 0.37 1.08 One-staged

The properties of the woven fabric were measured according to the following procedures. The results are summarized in Table 2, below.

-   -   Intrinsic Viscosity (I.V.): Each polymer was sufficiently         dissolved in ortho-chlorophenol at 120° C. to have a         concentration of 1%, and then measured therefor in a thermostat         of 30° C. using an Ubbelohde viscometer.     -   Melt Viscosity (M.V.): A polymer was sufficiently dried at         160° C. using a vacuum oven, and then measured therefor at         280° C. by use of a capillary type rheometer.     -   Crimp Ratio (T_(c), %): A yarn sample of 3000 denier in the         state of tensile force of 50 mg/denier being applied was treated         in boiling water (100° C.) for 20 min under a load of 0.5         mg/denier to the extent of each piece of the sample not being         tangled. Thereafter, the sample was allowed to stand for 24         hours under no load for natural dry. Subsequently, 1 min after         the sample was subjected to a load of 2 mg/denier, a length L1         was measured. In addition, 1 min after 200 mg/denier were         further applied to the sample under the load of 2 mg/denier, a         length L2 was measured. The measured values were introduced into         Equation 3, below, to determine a crimp ratio:         T _(c)(%)=(L2−L1)/L2×100  Equation 3     -   30% Elongation Elastic Recovery (FR₃₀, %): Three woven fabric         samples having sizes of 5.5×30 cm were prepared, and then         mounted in a width of 5 cm to a tensile tester so as to allow         the sample to be given under a preload. The sample was elongated         to have an elongation of 30% at 100%/min according to a         slow-rate elongation measurement (JIS L 1018-70), after which it         was shrunken at the same rate in a direction contrary to the         elongation direction. As such, the elongation (ε) was measured         when stress was to be a preload in a stress-elongation curve,         and averaged in warp and weft directions, and then introduced         into Equation 4 below.         FR ₃₀(%)=(30_(ε))/30×100  Equation 4

TABLE 2 Crimp Ex. Strength Elongation ratio FR₃₀ No. Spinnability (g/denier) (%) (%) (%) 1 ⊚ 3.30 23 42 86 2 ⊚ 2.65 29 65 97 3 ⊚ 2.49 35 60 93 4 ⊚ 2.78 23 68 95 5 X 1.90 35 75 95 6 ⊚ 1.63 37 70 97 Note: Spinnability, ⊚: excellent, ◯: good, Δ: normal, X: bad

As apparent from the above tables, when the viscosity difference is 2500 poise or more, the kneeling phenomenon occurs, thus spinning processability decreases. Further, as the intrinsic viscosity difference between PET and PTT increases, it is possible to manufacture a yarn having high crimp properties. Different from a bi-component spinning process using the regular PET polymer pairs with only the intrinsic viscosity difference, the one-staged spindraw process using PET & PTT can exhibit higher crimp properties than a two-staged process of comparative examples as mentioned below, and can be applied to manufacture the conjugated yarn having high crimping properties. In the cases of using the PET polymer having a low intrinsic viscosity of 0.46, it is difficult to set the spinning conditions, and the manufactured yarn is low in strength. Thus, when such a yarn is formed into a woven fabric through the dyeing and finishing process, the woven fabric suffer from low tear strength.

COMPARATIVE EXAMPLES 1-4

To offset the intrinsic viscosity difference between two types of polymers in extruders of a spinning machine of FIG. 1, an inclined circular spinnerette was used. A polymer A was exemplified by PET having different intrinsic viscosities, and a polymer B was exemplified by PET having intrinsic viscosity of 0.635 and PTT having intrinsic viscosity of 0.990, as represented in Table 3, below. 50% of the polymer A and 50% of the polymer B were spun to be a conjugated yarn at spinning temperatures of 280-290° C. through a one-staged process or a two-staged process. A cooling air of 23° C. at 5-120 cm directly under the nozzle was fed at 0.35 m/sec, and 0.5-1.1 wt % of a spinning oil was used. The prepared yarn was woven in warp and weft directions to obtain a woven fabric of 100 g/m², which was then dyed at 120° C. The woven fabric was measured for properties as in the above examples. The results are shown in Table 4, below. TABLE 3 Spinning Visco. C. Ex. Polymer Polymer Temp. Differ. throughput Interface Shape No. A B (° C.) (poise) Ratio Ratio Ratio Process 1 PET 0.550 PET 0.630 290 1200 5:5 0.21 1.04 Two-staged 2 PET 0.460 PET 0.635 285 1800 5:5 0.30 1.05 Two-staged 3 PET 0.550 PTT 0.99  280 2100 5:5 0.25 1.02 Two-staged 4 PET 0.460 PET 0.635 280 2000 5:5 0.34 1.03 One-staged

TABLE 4 Crimp C. Ex. Strength Elongation Ratio FR₃₀ No. Spinnability (g/denier) (%) (%) (%) 1 Δ 3.84 30 18 63.5 2 ⊚ 2.90 30 35 91.4 3 ⊚ 2.55 25 45 85 4 ⊚ 3.01 28 20 74.9

From the above results, it can be seen that the viscosity difference of 1500 poise or less results in low spinnability. Also, the larger the intrinsic viscosity difference between the two types of polymers, the higher the possibility of manufacturing the conjugated yarn with high crimping properties. Meanwhile, in the above tables, it is noted that crimp ratio by the one-staged spindraw process is lower than that by the two-staged process composed of spinning and drawing. (comparative example 2 & 4)

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a polyester conjugated yarn, and a manufacturing method thereof. The polyester conjugated yarn having a circular cross section can be manufactured even by a one-staged process using an inclined circular spinnerette, which is advantageous in terms of superior self-crimping properties. Further, the conjugated yarn can be applied to manufacture woven/knit fabrics exhibiting softness of the touch, beautiful colors, high drapery and bulk properties, due to inherent characteristics of polytrimethyleneterephthalate as a constitutive component thereof. Moreover, the polyester conjugated yarn can have high spinnability, no kneeling, and improved uniformity index.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing firm the scope and spirit of the invention as disclosed in the accompanying claims. 

1. A method of manufacturing a polyester conjugated yarn, comprising: subjecting two types of polymers having a large intrinsic viscosity difference to bi-component spinning in a spindraw process by use of an inclined circular spinnerette so as to cause the polymers to have a side-by-side sectional structure, the two types of polymers being polyethyleneterephthalate with intrinsic viscosity of 0.45-0.65 as a first polymer and polytrimethyleneterephthalate with intrinsic viscosity of 0.90-1.10 as a second polymer, wherein the polyester conjugated yarn has a cross section satisfying Equations 1 and 2, below: 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2 wherein, line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer; line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2 line EF: a maximum length of a long axis of a cross section of a yarn; line GH: a maximum length of a short axis of a cross section of a yarn.
 2. The method according to claim 1, wherein the two types of polymers have a melt viscosity difference not more than 2500 poise when being subjected to bi-component spinning.
 3. The method according to claim 1, wherein the polyethyleneterephthalate as the first polymer comprises terephthalic acid and ethylene glycol as constitutive monomers, and the polytrimethyleneterephthalate as the second polymer comprises terephthalic acid and propanediol as constitutive monomers, and the first and second polymers do not have an another functional component to be copolymerized.
 4. The method according to claim 1, wherein the polyethyleneterephthalate as the first polymer constitutes 30-70 wt %, and the polytrimethyleneterephthalate as the second polymer constitutes 70-30 wt %, based on total weights of the polyester conjugated yarn.
 5. The method according to claim 1, wherein the bi-component spinning uses a one-staged spindraw process, in which a first godet roller is set to a rate of 2000 m/min or more, and a second godet roller is set to a rate of 4000 m/min or more.
 6. The method according to claim 1, wherein the polyester conjugated yarn has a strength of 2.0-3.3 g/denier, and an elongation of 20-40%.
 7. The method according to claim 1, wherein the polyester conjugated yarn has a crimp ratio not less than 20%.
 8. A polyester conjugated yarn, comprising two types of polymers having a large intrinsic viscosity difference subjected to bi-component spinning so as to cause the polymers to have a side-by-side sectional structure, the two types of polymers being polyethyleneterephthalate with intrinsic viscosity of 0.45-0.65 as a first polymer and polytrimethyleneterephthalate with intrinsic viscosity of 0.90-1.10 as a second polymer, wherein the polyester conjugated yarn has a crimp ratio not less than 20% and a circular cross section satisfying Equations 1 and 2, below: 0≦(interface ratio=length of line CD÷length of line AB)≦0.6  Equation 1 1≦(shape ratio=length of line EF÷length of line GH)≦1.4  Equation 2 in which, line AB: a length of a long axis of an interface between a high viscosity polymer and a low viscosity polymer; line CD: a length of a short axis of an interface between a high viscosity polymer and a low viscosity polymer/2 line EF: a maximum length of a long axis of a cross section of a yarn; line GH: a maximum length of a short axis of a cross section of a yarn. 